Arc lamp and control circuit therefor



Patented Mar. 28, 1939 UNITED STATES PATENT OFFICE ARC LAMP AND CONTROLCIRCUIT THEREFOR Application April 12, 1938, Serial No. 201,550

11 Claims.

My invention relates to electric arc lamps and has particular referenceto a control device and system for accurately controlling the feeding ofcarbon electrodes employed in such lamps.

In carbon electrode arc lamps it is desirable to maintain the arc lengthas nearly at a constant value as possible in order to maintain a uniformdegree of brilliance of illumination therefrom and to maintain thequality of the light produced by the are, any appreciable variation inthe arc length not only altering the output of light from the are butalso changing the spectrum quality of the light.

In certain types of work, such as the lighting of motion picture sets,especially for color photography work, it is essential that the quantityof light be maintained at a uniform brillanceand any variation in thespectrum value of the light seriously affects the photographic results.It has been the common practice to employ high intensity arc lamps forthis purpose to produce as brilliant illumination as possible but thecharacter of control for the feeding of the carbons of such lamps hasbeen such that uniform arc length and uniform spectrum quality of thelight has been substantially impossible to obtain.

It is also essentialthat the mechanism employed to feed the electrodesmust be sufficiently sensitive in its response to variation in the arclength as to compensate for the burning away of electrodes in smallincrements to avoid sputtering or the creation of other noises whichwould interfere with. the recording of sound upon the set.

It is an object of my invention therefore to provide a carbon arc lampprovided with automatic electrode feeding apparatus employed to be sosensitive as to maintain the light output and the spectrum quality ofthe light at constant values.

Another object of my invention is to provide a multiple-arc lamp whichwill effectively utilize the maximum amount of the current passingtherethrough for the production of light and in which the set ofelectrodes for each arc is provided with its individual electrodefeeding apparatus so connected as to respond accurately and seiisithielyto minute variations in the length of the are between the associatedelectrodes.

Another object of my invention is to provide an arc lamp circuit fortwin arcs in which the electrodes for each of the arcs is provided withits individual feeding mechanism, so connected in Wheatstone bridgecircuits that the bridge circuit for one are and the bridge circuit forthe other are will contain a common leg.

Another object of my invention is to provide a control circuit formultiple-arc lamps in which a single ballast resistor is employ-ed incircuit with all of the arcs and in which control devices for feedingeach of the arcs are so connected in Wheatstone bridge circuits that theballast resistor constitutes a common leg in bridge circuit for each ofthe control devices.

Other objects and advantages of my invention will be apparent from astudy of the following specifications, read in connection with theaccompanying drawing, wherein Fig. l is a diagrammatic view illustratinga twin arc lamp, the feeding mechanism for feeding the electrodesthereof, and the circuit arrangements for including the ballast resistorin circuit with the arcs and the bridge circuits employed to supplycurrent to the feeding devices; and

Fig. 2 is a schematic View of a modified wiring arrangement which may beused for a plurality of arcs, either two or more.

Referring to the drawing, I have illustrated my arc lamp control circuitin Fig. 1 as applied to twin arcs A and B, the are A of which maycomprise a pair of electrodes A! and A2 adapted to be secured in anysuitable holders A3 and. A4, respectively. The holders A-3 and A-4 areso connected to feeding apparatus as to permit the ready feeding of theelectrodes AI and A2 toward and away from each other, such feedingapparatus being diagrammatically illustrated herein as including gearracks. A5 and A6 attached respectively to the holders A-3 and A--4, thegear racks A-5 and A-6 meshing with a pinion A! which is adapted to bedriven by the armature A8 of a small direct current electric motor AM,suitable reduction gearing being preferably interposed between thearmature A-8 and the pinion Al, as will be understood by those skilledin this art, but which, in order to avoid unnecessarily complicating thedrawing, has been omitted herein.

Similarly, the arc B may comprise a pair of electrodes Bl and B2,identical with the electrodes Al and A-2, adapted to be held in suitableholders B3 and B-4 to which are attached gear racks B-5 and B-6 meshingwith a pinion B'l driven by the armature 3-8 of a separate andindividual motor BM.

The circuit for supplying current to the arcs A and B may be termed thearc circuit and includes a ballast resistor R connected in seriescircuit relation with and disposed between the two arcs A and B so thatthe current supplied from a suitable source through conductors Ll andL-2 will extend from line conductor L-i through conductor 20, theelectrodes A2l A|, thence through the conductor 2 ballast resistor R,conductor 22, electrodes B-2 B--| and thence by way of conductor 23 toline con-- ductor L2.

The motor AM for feeding the electrodes A and A-2 toward and away fromeach other may be connected in a Wheatstone bridge circuit, two legs ofwhich are formed by the are A and ballast resistor R, respectively,while the other two legs of which are formed by an auxiliary circuitincluding resistor AR having a movable tap 24 adapted to connect withany point along the resistor AR. The resistor AR is illustrated as beingconnected between line conductor L-I and the far end 25 of the ballastresistor R as by means of conductors 26 and 21, respectively, so thatthat portion of the resistor AR which lies to the left of the tap 24will constitute one leg of the bridge circuit while that portion of theresistor AR which lies to the right of the tap 24 will constitute theother leg of the circuit. The armature A8 of the control motor AM forthe are A is preferably connected directly across the mid terminals ofthe Wheatstone bridge circuit so formed; that is, the armature A-8 isconnected directly between the near terminal 28 of the ballast resistorR and the tap 24 by means of conductors 29 and 30.

By reference to that portion of the circuit hereinbefore described, itwill be apparent that the motor armature A8 will receive current due toany unbalancing of the bridge circuit, the direction of flow of current(and thus the direction of rotation of the armature A-8) depending uponwhether the arc length has increased or decreased beyond a valuenecessary to balance the circuit.

Similarly, the ballast resistor R and the arc B constitute two legs of asecond Wheatstone bridge circuit, the other two legs of which are formedby a second auxiliary resistor BR having a movable tap 3| correspondingto the tap 24, the resistor BR being connected between the near terminal28 of the ballast resistor R and line conductor L-2 by means ofconductors 32 and 33, respectively, the portion of the resistor BR lyingto the right of the tap 3| constituting one leg of the bridge circuitwhile that portion which lies to the left of the tap 3| constitutes theother leg.

The motor armature B-8 is similarly connected between the mid points ofthis circuit, that is, between the far terminal 25 of the ballastresistor R and the tap 3|, by means of a pair or" conductors 34 and 35,respectively. Thus any unbalancing of this second Wheatstone bridgecircuit (ballast resistor R, are B and resistor BR) as by lengthening orshortening of the arc B, will cause current to flow in one direction orthe other through the armature B-8, causing the electrodes B-| or B4 tobe moved toward or away from each other.

The motors AM and BM should be selected with as low inertia value aspossible so that the armatures thereof may readily respond to relativelysmall currents passing therethrough. The field windings of the motor maybe selected of any sunitable strength to insure sensitive response tosmall currents passing through the armatures and should be constantlyenergized as by connecting the field windings A-9 and B9 of the motorsin permanent series relation across the source of supply line conductorsL-|, L-2, as by means of conductors 36 and 31, a suitable resistor 33being interposed in the circuit if desired to limit or regulate thecurrent supplied to the field windings.

As will be understood by those skilled in this art, it is the commonpractice in arc lamp circuits to determine the desired voltage andcurrent to be supplied to the are for eflicient operation thereof andthen to select a ballast resistor capable of absorbing and dissipatingthe remaining line voltage. By employing twin arcs connected as setforth herein, each of the arcs acts as a resistance for the other arc,reducing the amount of ballast resistance which would be required to beused with either of the arcs singly, thus per mitting substantiallytwice the amount of illumination to be produced by the same amount ofcurrent which would otherwise be required for the operation of a singlearc employing the same carbons and the same current density.

By employing the circuit for the arcs and the circuit for the armaturesof the control motors as set forth herein, extremely sensitive andaccurate control of the arc lengths of each of the arcs may beaccomplished to maintain each of the arcs at a uniform length and thusmaintain uniformity of quantity of light and quality of light producedthereby. For example, my system may be employed to be operated from apower supply line furnishing volts by selecting the ballast resistor Rto have a voltage drop of about 41 volts when employed with electrodesand are lengths in the arcs A and B such that a voltage drop of 37 voltsis impressed across each of the arcs. With this arrangement, theauxiliary resistor AR may be so selected as to carry approximately fourto six amperes of current, the tap 24 being moved along the resistor Runtil the resistance ratio of the portions of the auxiliary resistor ARon opposite sides of the tap 24 is just equal to the optimum or desiredresistance ratio of the arc A to the resistance of the ballast resistorR when the are A is at its most desirable length, the auxiliary resistorAR thus supplying length, the potential across the arc will just equalthe reference resistor potential and no current will flow through thecircuit for the armature A8.

Similarly, the resistor BR is selected to pass four to six amperes whenthe legs 4| and 42 thereof are so balanced in the circuit as to permitno current to flow through the armature B-8.

Any lengthening of the are A will increase the voltage drop in the leg Aof the bridge circuit described by the arc A with a corresponding dropin the current flowing through are A and resistor R, thereby causing alowering of the voltage drop across the terminals of the resistor R. Asthe voltage between conductor L-| and the tap 24 will remainsubstantially constant, an unbalanced condition will occur in the bridgecircuit, causing current to flow through the motor armature A-B in thedirection necessary to move the electrodes A| and A-2 toward each other.By maintaining the current flow through the resistor AR at a relativelyhigh value in proportion to the current required to ilow through thearmature A-El to achieve movement thereof, the current flowing throughthe armature will not appreciably change the IR drop in the leg 39 ofthe bridge circuit and accurate balancing of the legs 39 and 4|] of thebridge circuit is permitted, insuring that the only unbalancing oi thecircuit will occur by reason of variation in the length of the are A.

Similarly, in the bridge circuit described by the iii are B, resistor Rand legs 4| and 42 of the auxil-= iary resistor BR, any variation in thelength of the arc B will similarly cause the armature B-8 to respondsensitively thereto. I have found by actual experiments that a change involtage drop across the arcs A or B of from 1 to 1 volts is sufiicientto cause the armatures A-8 or B-8, respectively, to respond to suchchange and to feed the electrodes in the proper amount to maintain thevoltage drop across each of the arcs, respectively, within limits offrom 1 to 1 volts at all times during operation of the lamp, and I amthus enabled to maintain the spectrum quality of the lights produced byeach of the arcs at a constant value and to maintain the quantity oflight produced by the arcs within a limit of i2 It will be observed thatwhile I utilize twin arcs connected in series relation with each other,I provide two distinct Wheatstone bridge circuits, one for each of thearcs, though I employ the ballast resistor R as a common leg in each ofthe bridge circuits and I am thus enabled to employ the accuracy andsensitivity of a Wheatstone bridge control circuit with twin arcs.

In Fig. 2 I have illustrated a modified set of circuits which adapts mysystem for use with either two or more than two arcs, three beingillustrated in Fig. 2 as connected in an arc circuit across the supplyline Ll and L-2 with a common ballast resistor R in series therewith. Anauxiliary circuit is illustrated as including the control resistorsassociated with each of the arcs A, B and C, such circuit eitherincluding one long auxiliary resistor or a series of short resistors,one associated with each of the arcs, to provide a series of referencepotentials along the auxiliary circuit, one equal to the optimumpotential between the successive arcs so as to provide points ofconnection associated with each of the arcs across which the armaturesof the motors or other electrornotive feeding means may be connected insuch manner as to be supplied with current in correspondence withchanges in the potential across the associated arc. For example, asection of the long resistor or a short resistor designated by thereference character AR corresponds to the resistor AR illustrated inFig. 1 while another section of the long resistor or a short resistor tobe associated with the arc B is designated by the reference characterBR.

A similar section of the long resistor or a short individual resistor isassociated with the are C. by employing the auxiliary resistors AR, BRand CR connected as illustrated in Fig. 2, the motor armatures A8, B-8and -8 associated with the respective arcs A, B and C may be connectedas indicated in Fig. 2, the motor armature A-8 associated with the are Ahaving one of its terminals connected to the conductor 50 which extendsbetween are A and arc B, while the other of its terminals is connectedto an adjustable tap i engaging the resistor AR. Similarly the motorarmature B--8 associated with the arc B may be connected to conductor 52extending between arcs B and C, while the other of its terminalsconnects with a tap 53 engaging resistor BR, while the motor armature C8has one of its terminals connected to conductor 54 and the other of itsterminals connected to a tap 55 engaging the resistor OR.

in effect, such connection provides a plurality of bridge circuits, thebridge circuit associated with the are A including as one of its legsthe arc A, as another of its legs the remainder of the are circuitincluding arcs B and C and ballast resistor R, while the controlresistor AR constitutes a third leg of this bridge circuit and thecombined resistors BR and CR constitute the fourth leg.

As was described with reference to Fig. 1, the amount of current flowthrough the resistors AR, BR and CR should be selected of a relativelyhigh value in proportion to the amount of current required to flowthrough the motor armature A8, B8 or CB to cause these armatures toturn. Thus by properly selecting the resistance for the resistor AR suchthat when the are A is at its proper length, no current will flowthrough the motor armature A-8, and it follows that any lengthening ofthe arc A will increase the voltage drop in the leg A of its bridgecircuit with a corresponding drop in the current flowing through are Aand the remainder of the main circuit for are A (namely, arcs B and Cand resistor R), thereby causing a lowering of the voltage drop betweenconductor 50 and line conductor L-2. Since the voltage between line L--|and the tap 5| will remain substantially constant, an unbalancedcondition will occur causing current to flow through the motor armatureA-8 in the direction necesasry to move the electrodes of are A towardeach other.

Similarly, any change in the voltage drop across are E, as bylengthening of arc B, would cause an unbalance of the bridge circuit forare B (this circuit including as one leg arcs A and B, as another legarc C and ballast resistor R, as a third and fourth leg the sections ofthe auxiliary resistor AR lying to the right and to the left of the tap53) and hence current will flow through the motor armature B8 in adirection necessary to move the electrodes of arc B toward each other.

Similarly, any change in arc length of are C will unbalance its bridgecircuit (which includes as one leg arcs A, B and C', as another legballast resistor R, as the third and fourth legs the sections of theauxiliary resistor AR, BR and CR lying to the right and left of the tap55).

As will be understood by those skilled in this art, the field windingsfor the control motors for each of the arcs may be connected in seriesrelation across the supply line as indicated at A-9, B--9 and (3-9 inFig. 2.

With the form of the device illustrated in Fig. 2, it will be apparentthat the effect of a sensitive Wheatstone bridge circuit for each of thecontrol motors may be accomplished even though the several arcs to becontrolled are connected in series relation with each other and with acommon ballast resistor, each of the control motors functioning with itsassociated arc to automatically and sensitively respond to changes inthe length of the associated arc to correct for such changes and thus tomaintain each of the arcs at a constant length throughout the life ofthe electrodes. While I have illustrated only three arcs in Fig. 2, itwill be apparent to those skilled in the art that with proper voltagesimpressed upon the supply line any desired number of arcs may beconnected in series circuit relation, any variation in the length of oneof the arcs being corrected so instantaneously as to prevent suchfluctuation in one of the arcs from affecting the brilliance or spectrumvalue of the light produced by the other arcs.

While I have shown and described the preferred embodiment of myinvention, I do not desire to be limited to any of the details ofconstruction shown or described herein, except as defined in theappended claims.

I claim:'

1. In an arc lamp circuit, two sets of electrodes comprising two arcs, aballast resistor interposed in series relation with said arcs anddisposed in circuit between them, individual electromotive feeding meansfor each of said sets of electrodes for moving the associated set ofelectrodes to- Ward and away from each other, a control circuit for eachof said feeding means comprising a Wheatstone bridge circuit, the legsof one branch of which are formed respectively by the associated arc andthe ballast resistor, and the legs of the other branch of which areformed by auxiliary resistors, and means cross connecting theelectromotive means between the said branches.

2. In a system for controlling the feeding of the electrodes of twinarcs to maintain the length of each of said arcs at a substantiallyconstant value, an electromotive feeding means for each of said arcs, anarc circuit including a ballast resistor in series with and disposedbetween said arcs, and means connecting the electromotive meansassociated with each arc in a Wheatstone bridge circuit including theassociated are as one of the legs thereof, said ballast resistor beingcommon to both of said bridge circuits.

3. In an arc lamp circuit, two sets of electrodes comprising two arcs, aballast resistor interposed in series relation with said arcs anddisposed between them, individual electromotive feeding means for eachof said sets of electrodes for moving the associated set of electrodestoward and away from each other, a control circuit for each of saidfeeding means comprising a Wheatstone bridge circuit, the legs of onebranch of which are formed respectively by the associated arc and theballast resistor, and the legs of the other branch of which are formedby auxiliary resistors, and means cross connecting the electromotivemeans between the said branches, said auxiliary resistor for eachcircuit being of relatively low resistance as compared with theresistance of said feeding means whereby the current in said auxiliaryresistor will be relatively great as compared with the current requiredfor response by said electromotive means.

4. In an arc lamp circuit, two sets of electrodes comprising two arcs, aballast resistor interposed in series relation with said arcs anddisposed in circuit between them, individual feeding means for each ofsaid sets of electrodes including an electric motor, a control circuitfor each of said feeding means comprising a Wheatstone bridge circuit,the legs of one branch of which are formed respectively by theassociated arc and the ballast resistor, and the legs of the otherbranch of which are formed by auxiliary resistors, and means crossconnecting the armature of the motor between said branches.

5. In an arc lamp circuit, a plurality of sets of electrodes eachforming an arc, ballastresistor means interposed in series relation withsaid arcs and disposed between said arcs, individual feeding means foreach of said sets of electrodes including an electric motor, a controlcircuit for each of said feeding means, each of said circuits comprisinga Wheatstone bridge, the legs of one branch of which are formedrespectively by the associated arc and the ballast resistor, the legs ofthe other branch of which are formed by auxiliary resistors, and meanscross-connecting the armature of the motor between said branches, theballast resistor disposed between each pair of adjacent arcsconstituting a common leg in the bridge circuits associated with therespective arcs.

6. In an arc lamp circuit, two sets of electrodes comprising two arcs, aballast resistor interposed in series relation with said arcs anddisposed between them, individual feeding means for each of said sets ofelectrodes including an electric motor, means connecting the fieldwindings of said motors for constant predetermined excitation, a controlcircuit for each of said feeding means, each of said circuits comprisinga Wheatstone bridge, the legs of one branch of which are formedrespectively by the associated arc and the ballast resistor, and thelegs of the other branch of which are formed by auxiliary resistors, andmeans cross-connecting the armature of the associated motor between saidbranches, the ballast resistor constituting a leg common to both of saidbridge circuits.

7. In a system for so controlling the feeding of electrodes of aplurality of series connected arcs as to maintain the length of each ofsaid arcs at a substantially constant value, the combination of: an arccircuit extending across a supply line and including a plurality ofelements connected in series comprising a plurality of sets ofelectrodes for a plurality of arcs and a ballast resistor means; anauxiliary circuit connected across the supply line and includingselected re sistance portions associated respectively with each of saidarcs; individual electromotive feeding means for each set of electrodes;circuit means connecting each of said feeding means to one of theelectrodes of the associated are at a point in said arc circuit betweenthe associated arc and one of the remaining elements in the arc circuit;and circuit means connecting said feeding means to said auxiliarycircuit at a point between the associated resistance portion and oneother resistance portion.

8. In a system for so controlling the feeding of electrodes of aplurality of series connected arcs as to maintain the length of each ofsaid arcs at a substantially constant value, the combination of: an arccircuit extending across a supply line and including a plurality ofelements connected in series comprising a plurality of sets ofelectrodes for a plurality of arcs and a ballast resistor means; anauxiliary circuit connected across the supply line and includingselected resistance portions associated respectively with each of saidarcs; individual electric motors for feeding each set of electrodes;circuit means connecting the armatures of each of said motors to one ofthe electrodes of the associated are at a point in said are circuitbetween the associated arc and one of the remaining elements in the arccircuit; circuit means connecting said armatures to said auxiliarycircuit at a point between the associated resistance portion and oneother resistance portion; and means connecting the fields of each ofmotors in circuit across said supply line.

9. In a system for so controlling the feeding of electrodes of aplurality of series connected arcs as to rnaiiiitain the length of eachof said arcs at a substantially contsant value, the combination of: anarc circuit extending across a supply line and including a plurality ofelements connected series comprising a plurality of sets of electrodesfor a plurality of arcs and a ballast resistor means; an auxiliarycircuit connected across the supply line and including selectedresistance portions associated respectively with each of said arcs;individual electromotive feeding means for each set of electrodes;circuit means connecting each of said feeding means to one of theelectrodes of the associated arc at .a point in said arc circuit betweenthe associated arc and one of the remaining elements in the arc circuit;circuit means connecting said feeding means to said auxiliary circuit ata point between the associated resistance portion and one otherresistance portion, the resistance in said auxiliary circuit being lowrelative to the resistance of said feeding means.

10. In a system for so controlling the feeding of electrodes of aplurality of series connected arcs as to maintain the length of each ofsaid arcs at a substantially constant value, the combination of an arccircuit including a ballast resistor and a plurality of sets ofelectrodes connected in series relation with each other to comprise aplurality of arcs; individual electromotive feeding means for each ofsaid sets of electrodes for moving the associated set of electrodestoward and away from each other; an auxiliary resistor associated witheach of the arcs and connected in parallel with its associated arc andsaid ballast resistor; and means connecting one terminal of each of saidfeeding means between its associated arc and the ballast resistor andconnecting the other terminal of said feeding means to the auxiliaryresistor associated with that arc to supply said feeding means withcurrent in correspondence with variations in the resistance of theassociated arc.

11. In a system for so controlling the feeding of electrodes of aplurality of series connected arcs as to maintain the length of saidarcs at a substantially constant value, the combination of: an arccircuit extending across a supply line including a plurality of sets ofelectrodes connected in series to comprise a plurality of arcs; anauxiliary circuit connected across the supply line and includingselected adjustable resistance portions associated respectively witheach of said arcs; individual electromotive feeding means for each setof electrodes; circuit means connecting the feeding means for each arcto the arc circuit between the associated arc and the next adjacent arcand to the auxiliary circuit between the resistance portion for theassociated arc and the resistance portions for the remaining arcs; andmeans for manually adjusting the resistance of said resistance portions.

FIELDING C. COATES.

